Mazda3 2010 DTC P0172: Long Term Fuel Trim Issue After MAF Sensor Replacement

Hello, I'm from China and my English is not perfect, so sorry for any mistakes. I own a 2010 Mazda3 with a 2.0L engine and automatic transmission. Recently, I've been experiencing a DTC P0172 code (Rich Fuel Trim Bank 1). Initially, the Long Term Fuel Trim (LONGFT) was consistently at -20.31%, while Short Term Fuel Trim (SHORTFT) remained within acceptable range (< ±5%). I noticed that the Mass Air Flow (MAF) sensor readings were significantly off — showing 2.7 g/s in a sea-level city, which is not accurate for a 2.0L engine. After replacing the MAF sensor, the vehicle's performance improved: LONGFT stabilized at 0%, SHORTFT remained within ±5% range, and driving felt normal. However, after about one week, I noticed that LONGFT has started to drift again — now showing around +1% during operation. I'm concerned whether this indicates a lean condition or if it's safe to ignore as long as the value stays under ±5%. Today, while driving, I observed that LONGFT consistently reads between -3.9% and -2.3%, never going positive. At idle, it occasionally returns to 0% when braking, but often remains at -3.9% or -2.3%. When parked, the reading starts at -3.5%, then drops to -2.3% after about 20 seconds, and finally settles at -0.8% after another 20 seconds. The LONGFT value changes linearly over time — it does not oscillate between negative and positive as expected. I believe this is abnormal because the ideal behavior should be: negative → zero → positive → zero → negative (a balanced cycle). Currently, it remains in a consistently negative range. What part or component should I check now? Any advice would be greatly appreciated.